Hermetically sealed electromagnetic relays are used for switching of high electrical currents and/or high voltages, and typically have fixed and movable contacts, and an actuating mechanism supported within a hermetically sealed chamber. To suppress arc formation, and to provide long operating life, air is removed from the sealed chamber by conventional high-vacuum equipment and techniques. In one style of relay, the chamber is then sealed so the fixed and movable contacts coact in a high-vacuum environment. In another common style, the evacuated chamber is backfilled (and sometimes pressurized) with an insulating gas (e.g., sulphur hexafluoride) with good arc-suppressing properties.
The sealed chamber is conventionally formed by a glass or ceramic envelope which is fused (glass-to-metal seal) or brazed (ceramic-to-metal seal) to metal components of the relay such as terminal pins and a typically cylindrical or tubular metal base. These fused or brazed junctions are specified by Military Specification MIL-R-83725 with respect to high-voltage sealed relays.
Properly selected grades of glass or ceramic provide the essential characteristics of low gas permeability, excellent insulating or dielectric qualities, low outgassing, and mechanical strength. Glass envelopes, however, are handmade by skilled artisans, and are expensive and subject to breakage, and ceramic envelopes are both expensive to press and metalize, and difficult to procure. It is to the solution of these problems that our invention is directed.
Our improvement is directed to the replacement of these glass or ceramic chamber-enclosing envelopes with an inexpensive and easily formed vacuum-tight assembly of plastic and epoxy, or in an alternative form, an envelope made entirely of epoxy. We have established that this type of plastic/epoxy or epoxy envelope provides an excellent hermetic seal, good dielectric and outgassing characteristics, and a strong, inexpensive sealed relay for switching high currents and/or high voltage.
Attempts have been made in known designs to use plastic materials in relays, and U.S. Pat. Nos. 4,039,984, 4,168,480 and 4,880,947 are examples of the use of epoxy resins as adhesives to secure together relay housing components. Curing of the epoxy to a cross-linked thermoset state shrinks the joint bond and weakens the seal. Certain other designs (e.g., U.S. Pat. 5,554,963) have used thermoplastic (as opposed to cross-linked thermosetting) polymers, but the resulting relay envelope is not a true hermetic seal which can maintain either a high-vacuum or high-pressure environment.
For purposes of this invention disclosure, a hermetic seal means a seal which is sufficiently strong and impermeable to maintain for a long term a high vacuum of 10-.sup.5 Torr (760 Torr=one atmosphere) or less, and a pressure of at least 1.5 atmospheres. In contrast to the prior-art designs, the present invention achieves hermetic sealing by encapsulating the relay chamber in a jacket of impermeable epoxy or a comparable thermosetting polymer, the jacket having single-junction epoxy-to-metal bonds. Shrinkage of the epoxy during polymerization is a significant advantage in the invention as it provides a strong and reliable single-junction seal.
In one embodiment described below, an unsealed relay is encapsulated in a vacuum chamber, thus eliminating the need for an evacuation tube which characterizes prior relay designs. This same new method can be used to make pressurized relays which are evacuated, backfilled and encapsulated within a properly equipped chamber.